The present invention relates to vivapain-4 (VX-4), which is a cysteine protease of Plasmodium vivax, showing pH-dependent switching of substrate specificity. More specifically, the present invention relates to a method of treating a parasitic disease caused by Plasmodium vivax by inhibiting VX-4, a method of screening a protease inhibitor acting on VX-4, wherein the protease inhibitor is useful as an anti-malarial agent acting on Plasmodium species, for example, Plasmodium vivax, and a method of identifying the activity of VX-4.